With a recent development of micro-electronics, demands for minimizing a size, a weight and a thickness of a battery and a demand for increasing an energy density of it have been increasing.
A lithium secondary battery, which uses metallic lithium or lithium alloy as its negative active material, has become the object of public attention for its high energy density, and a lithium secondary battery using an liquid electrolyte is now put to practical use. However, the liquid electrolyte is apt to cause a solution-leakage to the outside of the battery and an elution or volatilization etc. of electrode material. For this reason, there have been problems, in the lithium secondary batteries, of worse long-term reliability and poor safety due to flying-around of electrolyte solution during a sealing process.
Study and development of the lithium secondary battery using the solid polymer electrolyte have been carried on actively in recent years. This battery has such advantages that it is provided with a solution-leakage resistance and does not produce an elution or volatilization etc. of electrode material because it uses the solid polymer electrolyte in addition to its feature of high energy density. Namely, the lithium secondary battery using the solid polymer electrolyte has a high energy density and is excellent in safety and long-term reliability.
However, the above-mentioned lithium secondary battery using the solid polymer electrolyte has included the following problems.
(1) Lithium forming a negative active material grows into a branched shape on a surface of negative electrode at time of charging, i.e. a dendrite of lithium has been formed, so that an internal short-circuiting has been produced due to contact of it with a positive electrode or the lithium has deposited into a mossy shape to cause a falling-off of lithium. As the result, the charge/discharge cycle life is extremely shortened. The formation of dendrites is caused by the fact that the lithium turns into ions at the time of discharging and is eluted to make the negative electrode surface into a corrugated shape and that the lithium deposites on convex portions in a concentrated manner at time of charging after the discharging. The deposited lithium has a high activity because it is composed of fine particles including large surface areas, so that it reacts with the organic electrolyte solution and dissolves the electrolytic solution to deteriorate the electrolyte so as to shorten the charge/discharge cycle life. In order to cope with this problem, the use of lithium alloy as the negative electrode is well known (Published Patent Application (KOKAI) No. 52-5423, for example). However, the strength of the lithium alloy is small as represented by lithium-aluminum alloy, so that cracking or breaking into fine particles may sometime occur due to repeated charging and discharging operations. Therefore, the charge/discharge cycle life has not heretofore been improved satisfactorily. PA0 (2) The solid polymer electrolyte generally used so far is a high-molecular compound obtained by polymerizing a polyether which has included no functional group at an end of a principal chain and included an ionic salt, so that its electric conductivity has been small. Therefore, in the conventional lithium secondary battery, there has been a problem in that it has been hard to obtain a discharge performance to an extent durable for practical use. To cope with this disadvantage, a solvent serving as a plasticizer has been added to the solid polymer electrolyte. When the solvent is added, an electric conductivity of the solid polymer electrolyte can be improved but its strength is weakened. When the strength is weakened, the solid polymer electrolyte is apt to be damaged so as to cause an internal short-circuiting etc. In the solid polymer electrolyte added with solvent, the formation of dendrites of lithium becomes larger than that with no solvent, and a pressure applied to the assembled element specially required for the positive electrode side can not be obtained because of a low resiliency as compared with the solid polymer electrolyte added with no solvent, so that there has been a problem of short charge/discharge cycle life. PA0 (1) This invention provides a lithium secondary battery comprising a positive electrode, an electrolyte layer composing of a solid polymer electrolyte, and a negative electrode using metallic lithium or lithium alloy as an active material, characterized by that the electrolyte layer comprises at least two layers in which one layer in contact with the negative electrode is composed of an electrolyte hard to react with the negative active material and the other layer is composed of an electrolyte easy to react with the negative active material. PA0 (2) The other layer of the disclosure (1) is so devised that this layer is easy to react with the negative active material because it includes a solvent, and the one layer of the disclosure (1) is so devised that this layer is hard to react with the negative active material because it includes a small content of solvent as compared with that of the other layer or it does not include the solvent at all. PA0 (3) The electrolyte of the one layer in the disclosure (1) is a high-molecular compound which is formed by polymerizing a polyether and does not have the functional group at the end of a principal chain but includes an ionic salt. The electrolyte of the other layer in the disclosure (1) is a high-molecular compound which is formed by polymerizing a polyether, has the functional group at the end of a principal chain and includes an ionic salt. PA0 (4) The electrolyte of one layer in the disclosure (1) is the same as that in the disclosure (3), and the electrolyte of the other layer of the disclosure (1) is composed of an electrolyte in which a material reactive with the negative active material is added to the electrolyte of the one layer.
An object of this invention is to provide a lithium secondary battery which has a high energy density, is excellent in safety and long-term reliability, and has a strength and charge/discharge cycle life durable for practical use.